The history of separating gas constituents from natural gas is a relatively recent one. In the 1940's the first prototype natural gas cyrogenic turboexpander was built and in the 50's this turboexpander concept was applied to air plants, hydrogen plants, and helium purification plants. However, it wasn't until the 1960's that the first commercial natural gas turboexpander plant started operation. As commercial demand for these separated gases increased, many other such plants came into existence each with better and improved designs for separating the lighter elements (methane, ethane, and propane) from the heavier elements (butane, pentane and their iso-components) contained in natural gas.
Many patents exist pertaining to these improvements with some of the more relevant patents being those to Gulsby (U.S. Pat. Nos. 4,464,190 and 4,453,958), Horton (U.S. Pat. Nos. 4,410,342 and 3,398,543), and Campbell, et al. (4,278,457 and 4,171,964). While each of these patents are improvements over their predecessors, none of them address the commercial need for nearly 100% gas separation and recovery.
It is an object of this invention to provide a method of achieving controlled separation of the natural gas feed stream constituents. It is another object of this invention to utilize a distillation column and recycled streams from other parts of the plant to affect the chemical equilibrium in the top section of the column for most economical product recovery. Another object of this invention is to employ a return condensate line feeding a demethanizer column to more favorably alter the chemical equilibrium in this demethanizer column.